So-called flat display devices, typically liquid-crystal displays and including organic EL displays and plasma displays, have been significantly widespread. Among other things, in small-sized portable electronic devices, liquid-crystal displays are used for most of display devices. One reason for adopting a flat display device is its light weight, compared with, for example, a plasma display with discharge gas contained in heavy glass.
However, the liquid-crystal display is of a transmission type not emitting light by itself, unlike a plasma display of a self-light-emission type. Therefore, in consideration of use in a bright location such as outdoor use during the daytime, a light source (a surface light source provided on a rear surface of the display: a so-called backlight) is required. However, the presence of the backlight causes an inconvenience, particularly in a portable electronic device, because power consumption by the backlight degrades the battery life.
By contrast, the organic EL display is of a self-light-emission type, and therefore does not require a backlight and, what is more, is light in weight. Therefore, the organic EL display is expected as a replacement for the liquid-crystal display. However, complete replacement has not yet been done, because organic molecules for use as a light-emitting material are disadvantageously vulnerable to oxygen and moisture and have a short life.
Therefore, superiority of the liquid-crystal display remains the same for the time being, and thus a solution to the problem described above will address social demands.
A patent document 1 mentioned below discloses a technology capable of reducing power consumption of a backlight (hereinafter, a conventional technology). In this technology, a touch sensor is provided on each of both side surfaces of the casing of a portable electronic device, and switch off control is performed on the backlight in response to a signal from each of the touch sensors. Here, “both side surfaces of the casing” means surfaces to be touched by a fingertip or others when the casing is held by hand. Therefore, according to this conventional technology, power control can be made in a manner such that the backlight is lit up when the casing is held by hand and the backlight is switched off when the hand is lifted. With this, unnecessary lighting of the backlight can be prevented to reduce power consumption of the backlight.